This invention relates generally to composite stray field electrode materials. More particularly, it relates to stray field electrodes comprising filled conductive silicone elastomers and to methods for determining the presence of water in insulating materials, such as those used in electrical power generation.
Water detection methods are available for a variety of end use applications. For example, the water content of porous materials such as soil has been measured by using electromagnetic fields, as described in U.S. Pat. No. 5,442,293. Moreover, the determination of moisture content in materials such as wet or cured concrete is sometimes very important in the building industry. U.S. Pat. No. 3,870,951, for example, describes an electrical measuring probe useful for such a purpose.
Water detection is also important in water-cooled electrical generators. The stator yoke in these generators surrounds the armature core and partially encloses the armature windings, which are sometimes referred to as "stator winding" or "stator bar." As one typical example, copper conductors are usually wound in the armature to form loops. The armature windings are arranged in such a manner that the desired voltage and current characteristics can be maintained by the generator in operation. A number of the individual conductors (sometimes referred to herein as "strands") inside the stator bars are hollow, to allow for the flow of cooling water from a coolant system.
Electrical insulation is wrapped around both the strands and the stator bars, and is also often used to separate some of the strands from each other, or from other conductive structures, such as portions of the steel stator yoke. The ground wall insulation which is usually wrapped around the stator bars can be formed of various materials. Examples are fiberglass tape, vacuum/pressure-impregnating resins, casting and potting resins, and different types of laminates made by bonding layers of a reinforcing web.
A preferred type of generator insulation is mica-based insulating tapes. Various types of mica-based tapes are available (e.g., Micapal.TM. tapes). Most of these types consist of mica flakes or laminates bound together with a resinous binder, such as an epoxy material.
The durability and integrity of the insulation during operation of electrical generators is of great importance. The stator bars operate at very high voltages, e.g., greater than 10,000 volts in a large generator. The voltage must remain isolated from ground. Any "flashover" from one stator bar to another, or from one electrical phase to another, or to ground could activate safety mechanisms which automatically shut down the generator. A sudden shut-down could instantaneously direct the current flow (often greater than 2,000 amps) to ground, an event which in some circumstances could severely damage many of the generator components.
The leakage of water into the ground-wall insulation can damage water-cooled electric generators and ultimately lead to the catastrophic failures mentioned above. Water leaks from the coolant system are often found in or near the many brazed connections at the junction of a stator winding terminus and a water hose connection. The leaks are caused by a variety of factors, such as stress cracks or porosity in copper castings or corrosion of the braze materials. As described by J. Timperley in Rotating Machinery, 62 PAIC 95 (copyright 1995 Doble Engineering Co.), water can then begin migrating along voids between the ground wall insulation and the strands, and can delaminate the mica-flake tape layers within the ground wall insulation. Failure of the generator can occur when water contaminates the ground-wall insulation in the vicinity of the stator core, where higher voltage stresses are present. Although on-line failures of generators due to water leakage are a rare occurrence, the damage caused by such an event could be extreme, as mentioned above.
Failure is most often experienced during routine maintenance or testing of the generator. For example, a stator water pumping unit may be left in operation when the generator is degassed. Under those conditions, the pressure differential may force water through a leak site and into the ground-wall insulation. In general, even very small water leaks can be detrimental to a generator if they are allowed to persist.
There are a number of techniques which are presently used to detect water in electrical insulation. One technique is capacitance mapping, as described in the Timperley article mentioned above. Another technique is the so-called direct current ("DC") technique for detecting water "trees" in insulated power cables, described in H. Oonishi et al. in IEEE Transactions on Power Delivery, Vol. PWRD-2, No. 1, January 1987. Other techniques also include the stator leak monitoring system (SLMS), use of a tracer gas such as sulfur hexafluoride, or the use of thermographic video cameras.
However, all of the above techniques suffer from one or more disadvantages. The disadvantages include poor sensitivity to the presence of water, inability to detect water while the generator is on-line, failure to accurately indicate the location of the water in the insulation, and damage to the insulation being monitored.
Thus, a method and an apparatus for detecting the presence of water in materials which overcomes or reduces these disadvantages would be desirable. It is desirable that an electrode, method and apparatus be suitable for testing of an insulating material which is incorporated into electrical power equipment, such as water-cooled generator insulation. It would be desirable if the apparatus could be employed while the power equipment was in operation, so that unnecessary shut-downs could be avoided. Finally, the method and apparatus should be accurate, exhibit good sensitivity to the presence of water and not add significant expense to any of the related procedures, such as power generation. The present invention is directed to overcoming or at least reducing one or more problems set forth above.